Rolling mill and  control system



March 17, 1964 K. NEUMANN ROLLING MILL AND CONTROL SYSTEM 'l \I I I u l 5 Sheets-Sheet 1 F762 UEW [M p a w m b X 9 n w 4/ I 7 m Filed Nov. 4,

March 17, 1964 NEUMANN ROLLINGMILL AND CONTROL SYSTEM 3 Sheets-Sheet 2 Filed Nov. 4, 1960 .QIIIIIIIIIIIIIIIIIIVI March 17, 1964 K. NEUMANN ROLLINGMILL AND CONTROL SYSTEM Filed Nov. 4, 1960 3 Sheets-Sheet 3 f Ii NH l 1| Q a 1.1 5 v I15 \Q s w N M @RR sazassz ROLUNG MILL ANB CONTROL SYSTEM Karl Neumann, St. lngbert, Saar, Germany, assignor to Mueller & Neumann G.rn.b.l-l., St. Ingbert (Saar), Ger- This invention relates to' metal working and more particularly to a rolling mill, including a control system for continuously and rapidly adjusting the roll gap in order to maintain the accuracy of rolled work pieces.

Heretofore rolling mills have been provided with mechanical means, such as adjusting screws and the like for adjusting the roll gap and such adjusting means have been operated either manually or by power means in order to control the cross sectional size of rolled work pieces. Such mechanical adjusting means has been entirely satisfactory for preliminary adjustment of the roll gap and has also been satisfactory for making adjustments where changes in the roll gap need not be relatively rapid. These mechanical adjusting devices do not conveniently permit relatively rapid adjustment, since such mechanical adjusting devices inherently include relatively large friction and inertia conditions which must be overcome in adjusting the roll gap. These conditions present no particular problem where the adjustment can be made at leisure during stoppage of the rolling mill, but the provision of continuous and relatively rapid adjustment during operation of the rolling mill has, with present me chanical adjusting devices, presented a problem which had not heretofore been solved satisfactorily.

In an attempt to solve this problem of continuously maintaining the accuracy of rolled work pieces by adjustment of the rolling pressure during operation of the rolling mill attempts have been made to utilize electronic control circuits and while such circuits have materially improved, the rapidity and accuracy with which changing conditions can be detected or anticipated in order to make compensatory adjustments, nevertheless, the use of such electronic control systems has not resulted in improving the mechanical adjusting devices with which such systems have been utilized and accordingly, no significant improvement has been made and the full capability of electronic control systems for this purpose have not been utilized.

It has also been heretofore proposed to substitute hydraulic adjusting devices for the mechanical adjusting devices previously utilized and theoretically at least, such hydraulic adjusting devices should be superior to the mechanical adjusting devices, since frictional forces, such as those developed in the threads of adjusting screws, are eliminated by the use of such hydraulic means. While hydraulic adjusting devices have been utilized to a limited extent for a long period of time, nevertheless, such devices have not found widespread application possibly because of the fact that, they were located within the course of rolling pressure and that since they are subjected to full rolling pressure relatively high fluid pressures must be utilized and under these conditions, the compressibility of the hydraulic fiuid utilized materially affects the accuracy of the adjustment and compensating for such compressibility has proved particularly difficult, since this is not a constant condition, but continuously changes in relation to changes of the hydraulic pressure. Accordingly, hydraulic adjusting means even combined with electronic control systems as heretofore applied have not satisfactorily solved the problem of continuously and accurately maintaining tolerances in rolled work pieces.

There are many factors which govern the accuracy of 3,,l24,982 Patented Mar. 17, 1964 rolled work pieces, some of these being the initial adjustment of the roll gap or the distance between the rolls, changes in temperature of the work pieces, changes in temperature of the rolls and associated supporting structure, bending or distortion of the rolls or associated supporting structure, wear of the rolls or associated supporting structure and shock caused by successive insertion of work pieces between the rolls. Some of these conditions are predictable and can be adequately compensated for, while others are unpredictable and variable and can only be properly compensated for during the rolling operation and consequently, in order to maintain a high degree of accuracy in the rolled work pieces, it is necessary to provide an automatic and rapidly operating control system which will anticipate or detect changing conditions which adversely affect the accuracy of the work pieces and to automatically and accurately adjust the rolling mill in order to compensate for such changing conditions in a manner which will continuously maintain the accuracy of the rolled work pieces.

A significant advance in the art of providing accuracy in rolling mills was provided by the inventions disclosed and claimed in US. Patents 2,934,968 and 2,934,969, to Karl Neumann et a1. and Karl Neumann respectively, issued May 3, 1960, and in US. application Serial No. 844,229 to Franz Blinn, filed October 5, 1959, which disclose the use of hydraulically pre-stressed stay bolts for absorbing the forces created by the rolling pressure and in these patents, a stay bolt is elongated by means of hydraulic pressure Within the elastic limit of such stay bolt to pre-stress the same, but only in order to pro-stress the upper and lower parts of a rolling mill frame associated therewith. It was found to be impractical to directly pre-stress closed frames of rolling mills because of the relatively large masses involved and also because of the difiiculty in providing uniform physical characteristics throughout the entire extent of the frame castings and accordingly, the use of pro-stressed stay bolts proved to be very beneficial, in that the physical properties of such stay bolts could be very accurately maintained and the problems of elongating such stay bolts within the elastic limit to pre-stress the same were relatively simple. While several methods of hydraulically elongating stay bolts are possible, one method as shown in the above mentioned patents includes the provision of a hollow stay bolt which provides a cylinder for receiving a slidably mounted piston with one end of the piston engaging an end of the stay bolt and with hydraulic pressure being introduced between the opposite end of the piston and the opposite end of the stay bolt. Consequently, by the introduction of fluid pressure, the stay bolt is elongated and such elongation may be very accurately controlled by the degree of fiuid pressure introduced.

It is accordingly an object of the invention to provide a rolling mill and control system which will operate to rapidly and continuously adjust the rolling mill to maintain the accuracy of rolled work pieces.

A further object of the invention is the provision of a rolling mill and control system which will operate in conditions which will adversely affect the accuracy of rolled work pieces and to continuously and rapidly compensate for such changes.

A still further object of the invention is the provision of a rolling mill and control system which will anticipate changes of conditions which will adversely afiect the accuracy of rolled work pieces and to compensate for such conditions.

Another object of the invention is the provision of a rolling mill and control system which eliminates the necessity for overcoming relatively large frictional and inertia forces, thereby permitting rapid and continuous adjustment of the rolling mill in order to maintain the accuracy of rolled work pieces.

A further object of the invention is the provision of a rolling mill and control system in which the roll gap is initially roughly adjusted, and thereafter continuously and rapidly adjusted by the elongation or contraction of pre-stressed stay bolts within the elastic limit of such stay bolts.

A further object of the invention is the provision of a rolling mill and control system which eliminates the necessity for overcoming relatively large frictional and inertia forces during adjustment of the rolling mill and which accomplishes such adjustments by the elongation or contraction of pre-stressed stay bolts within the elastic limits of such stay bolts, such elongation or contraction being accomplished by the use of controlled hydraulic pressure.

A still further object of the invention is the provision of a rolling mill and control system in which the rolling pressure is hydraulically adjusted by means of the elongation or contnaction of stay bolts and in which such hydraulic pressure is controlled in response to changes in the rolling pressure.

A further object of the invention is the provision of a rolling mill and control system in which the rolling pressure is controlled by the elongation or contraction of stay bolts under hydraulic pressure, the control system including a device which compares the sum of the actual rolling pressure and hydraulic pressure with a predetermined stressing value for the stay bolts in order to keep such stressing value and thus the roll gap constant.

A still further object of the invention is the provision of a rolling mill and control system in which the rolling pressure is hydraulically adjusted by means of the elongation or contraction of stay bolts, the cont-r01 system including a device which compares the sum of the actual rolling pressure and the hydraulic pressure with a predetermined stressing value for the stay bolts in order to keep such stressing value constant and which also includes a second device which corrects the pre-set or predetermined stressing value in accordance with changes in the actual rolling pressure in order to increase or diminish the actual rolling pressure for maintaining accuracy in the rolled work pieces.

Another object of the invention is the provision of a rolling mill and control system in which the rolling pressure is hydraulically adjusted by means of the elongation or contraction of stay bolts and in which the control system includes a plurality of photo-electric devices disposed in advance of the rolls to detect the approach of work pieces. entering the rolls and control circuits associated with the photo-electric devices to pro-adjust the roll pressure by contracting the stay bolts to compensate for the shock occurring upon entry of the work pieces between the rolls, the distance between the rolls and the photoelectric devices being chosen so as to compensate for the time delay in the response of the control system.

Further objects and advantages of the invention will be apparent from the following description taken in conjunction with the accompanying drawing wherein:

FIG. 1 is a diagrammatic elevational view with parts broken away and in section for greater clarity and showing a rolling mill, including upper and lower rolls and in which mechanical adjusting means is provided for the upper roll and also including a stay bolt which may be elongated or contracted within the elastic limits to adjust the rolling pressure;

FIG. 2 a view similar to FIG. 1, but showing a mechanical adjustment for the lower roll and a modified means for elongating or contracting the stay bolt;

FIG. 3 a diagrammatic view with the parts of the rolling mill in section and including means for adjusting the spacing between the upper and lower roll and also including a diagrammatic showing of a control system for con- 4 trolling the elongation or contraction of the stay bolt to maintain the accuracy of rolled work pieces,

FIG. 4 a diagrammatic view with a frameless rolling mill according to FIG. 3 and a hydraulically operated control system for controlling the elongation or contraction of the stay bolts to maintain the accuracy of rolled work pieces; and

FIG. 5 a diagrammatic view showing in further detail, the circuit and components of an electronic control system for use with this invention as shown in FIG. 3.

With continued reference to the drawing, there is shown in FIG. 1 a rolling mill, including a lower frame memher 1 andan upper frame member 2 connected by stay bolts 3'. The lower frame 1 carries a roll hearing or chock 4 which serves to rotatably support the lower roll 5 and supported from the upper frame member 2 is a roll hearing or chock 6 which serves to rotatably mount the upper roll '7. The position of the upper roll '7 and bearing a with respect to the lower roll 5 may be mechanically set by means of an adjusting screw 8 threadedly received in the upper frame member 2. The stay bolts 3 include enlarged lower ends 3a which engage the lower roll frame 1 and nuts 9 are threadedly received on the upper ends of stay bolts 3 and engage the upper frame member 2 to limit movement of the upper frame member 2 away from the lower frame member 1.

The enlarged lower portion 3a of the stay bolts 3 is hollow to provide a cylinder 12 and slidably disposed within the cylinder 12 is a piston 10 to which is attached a push rod 11 extending upwardly throughv the hollow stay bolt 3, the push rod 11 contacting the closed upper end of the stay bolt 3. The cap or upper frame member 2 is, according to the invention, normally main tained spaced a distance X from the lower frame member 1 by means of a yieldable device 13 which may take the form of a compression spring or hydraulic means and such yieldable means is so proportioned as to overcome the total weight of the upper frame member 2 and elements carried thereby to maintain the spacing X. The operation of this form of the invention to vary the rolling pressure will be later described.

With particular reference to FIG. 2, there is shown a somewhat modified form of rolling mill: and stay bolt structure in which lower frame member 1a serves to support a lower roll bearing or chock 4a which in turn rotatably carries a lower roll 5a and the position of the lower roll 5a may be setby means of an adjusting screw 8a threadedly received in the lower frame member la. An upper frame member 6a serves to rotatably support an upper roll 7a.

A solid stay bolt 14 extends through the upper and lower frame members 1a and 6a and the stay bolt 14 is provided at the upper end with a nut 9a engaging the upper frame member 6a and at the lower end with a nut 15 engaging the lower frame member 1a and the upper and lower frame members 1a and 6a are normally spaced apart a distance X. The lower frame member in is provided adjacent the upper end with an annular cylinder 12a surrounding the stay bolt 14a and slidably received within the cylinder 12a and surrounding the stay bolt 14 is a tubular piston 16 which projects above the upper end of the lower frame member 1a and is provided with a flange or abutment 16a engaging the lower end of the upper frame member 6a. The operation of this form of the invention to adjust the rolling pressure by elongation or contraction of the stay bolt 14 will be later described.

With particular reference to FIG. 3, there is shown a frameless rolling mill structure, including an upper chock Zil" rotatably mounting an upper roll 20a and a lower chock 21 rotatably mounting a lower roll 21a. The upper and lower chocks 20. and 21 are connected by a hollow stay bolt 3a provided at the lower end with an enlarged portion 3b enclosing a cylinder 12a in which is slidably received a piston Etta. A push rod 11a attached to the piston a engages the upper closed end of the stay bolt 3a. Provided on the upper end of stay bolt 3a is a nut 9b and disposed between the nut 9b and the upper frame member 20 is a pressure measuring device 26, the purpose and operation of which will be later described. The measuring device 26 may be of a hydraulic, mechanical or electrical nature.

In order to adjust the spacing X between the upper chock 20 and lower chock 21, there may be provided a nut 22 concentrically received on the stay bolt 3a in engagement with the upper chock 20 and threadedly engaging the nut 22 is a threaded member 23 keyed to the stay bolt 3a by means of a key 23a. The threaded member 23 may be rotated by means of a drive wheel 24 which may be releasably coupled to the nut 912 by means of a pin 25. The weight of the upper chock 21 roll 211a and associated parts is counter-balanced to maintain the upper chock 20 normally spaced a distance X from the lower chock 21 by yieldable means 13a engaging the lower chock 21 and the adjusting member 23 and the yielding means 13a may be in the form of a compression spring or, hydraulic means. The cylinder 12a on the lower end of the stay bolt 3a may be connected through a conduit 34 to a fluid pressure adjusting means 38 which in turn is connected to a source of fluid pressure 39.

An electronic control system for controlling the fluid pressure supplied to the cylinder 12a of the stay bolt 3a may well comprise a suitable electronic computer 32 connected through a visual indicator 31 by means of a conductor 30 to the pressure measuring device 26 and also connected through a visual pressure gauge 33 with the fluid pressure conduit 34. Also connected to the computer 32 is an adjusting device 43 and a second suitable electronic computer 44 is connected to the pressure measuring device 26 with the output of the computer 44 being connected to the adjusting device 43. The output of the computer 32 is connected to a suitable visual indicator 41 and also through a conductor 35 and switch 36 with a visual indicator 37 which in turn is connected to a switch 42 with the fluid pressure adjusting means 38. Photoelectric devices 45, 46 and 47 are positioned in the path of movement of work pieces entering the rolls of the rolling mill, such photo-electric devices 45, 46 and 47 being disposed in advance of the rolls and such photoelectric devices are connected to photo-electric control circuits 48, 49 and 50 which in turn are connected to the visual indicator 37 and through the switch 42 to the fluid pressure adjusting means 38. The electronic computers, as well as the visual indicating means, the photo-electric devices and the photo-electric control circuits are entirely conventional in nature and the adjusting means 43 connected to the computer 42 and the fluid pressure adjusting means 38 are also conventional in nature.

With particular reference to FIG. 5, there is shown in greater detail, the control system illustrated in FIG. 3. It may be seen that the pressure measing device 24 for determining the rolling pressure operates with stretch measuring strips which are arranged as resistors in a bridge circuit. A voltage which is proportional to the rolling pressure flows through conduit 30, the indicating device 31, three switches 125, 126 and 127 and through a resistor 128 to a grid 129 of tube 139. Tube 1311, as well as a second tube 131 are connected to the primary coil of a transformer 132 so that the secondary coil of the transformer 132 does not supply current when currents of equal magnitude are flowing in both tubes. The tubes 130 and 131 and transformer 132, as well as two rectifiers 133 and 134 in the conductors 35 and 35a through which the control voltages are conducted represent the control member 32 of FIG. 3.

The currents flowing in the conductors 35 and 35a are conducted to the coil of electro-magnet 135 in the fluid pressure adjusting means 38. The coil enters into an annular groove of a permanent magnet 136. The control piston 137 of a hydraulic relay 138 is connected to the coil core of the electro-magnet 135. Two springs 139 and 1419 yieldably hold the control piston 137 in a neutral position as illustrated. If a current flows through the coils of the electro-magnet 135, the control piston 137 is moved, depending on the direction of the current, either in opposition to spring 139 to the right or in opposition to spring 140 to the left. In the first instance, fluid pressure conduit 34 which leads to the pressure chambers 12a for the purpose of tensioning the stay bolt 3a as shown in FIG. 3, is connected to the conduit 33 from a fluid pressure source, while in the other instance, the pressure conduit 34 communicates with conduit 63.

In order to determine the hydraulic pressure in conduit 34, the indicating device 33 contains a pressure measuring means with resistors arranged in a bridge structure through which the hydraulic pressures are converted into electrical values. The bridge in the device 33 is open toward the output side. The two output conductors 141 and 142 are connected to a control or variable resistor 143 by which the load of grid 144 and thus the work point of the tube 131 may be preset. To that extent, the control resistor 143 is the adjusting device 43 for the control value de sired, because the magnitude of the currrent flowing through tube 131 determines, in the case where no rolling pressure is present and where very little or no current flows through tube 131 a maximum current in the conductors 35 and 35a to the adjusting means 38 in order to build up a pressure in pressure conduit 34. Depending on the value of the preset voltage in grid 144 of tube 131, the current in tube 131) will, due to the increasing hydraulic pressure or the hydraulic stretching force in the stay bolts, pass through the bridge circuit of the pressure measuring device 33 and will reach zero or a magnitude which is equal to the current in tube 1311 whereby zero current will be induced in the secondary coil of the transformer 132 and the control piston 137 of the hydraulic relay 138 will return to neutral position. In this manner, the maximum hydraulic stretch force is set in the stay bolt, namely, the desired control value which is to be maintained constant when rolling pressures rise.

When the equilibrium condition of the two currents from tubes 130 and 131 is disturbed due to the occurrence of a rolling pressure which opens the grid 129 of tube 130, a current flows in lines 35 and 35a in a direction which causes a movement of the control piston 137 toward the right until by operation of the pressure measuring device 33, grid 144 of tube 131 receives a positive charge to the point where equal currents flow in both tubes and transformer 132 is rendered inoperative.

The adjusting device 44 for compensating for the bending of the rollers consists of a control or variable resistor 145 which influences the charge of a second grid 146 of tube 130. As already explained, the bending of the rollers is to be compensated for, in that the hydraulic stretching force in the stay bolts is decreased by more than the magnitude of the rolling pressure, so that they are reduced in length and consequently, the bent rollers are set more closely relative to their bending action. This is obtained by reason of the fact that grid 129 becomes more positive, due to an increasing rolling pressure and as a larger current flows through tube 130, the grid 146 also becomes more positive and the current flow through tube 131] increases to an extent providing a guide magnitude depending on the rolling pressure. An equilibrium condition between the currents of the two tubes 130 and 131 will thus take place only at a corresponding lower hydraulic stretching force that amounts to a closer setting of the rollers.

FIG. 5 shows additionally a circuit arrangement for three photo-cells 45, 46 and 47, which in the case of a mill having three paths of movement for work pieces through the rollers, serve to initiate operation of the control circuit described above at the moment of movement of the leading work piece into range of the associated photo-cell. The photo-cells actuate photo-electric relays 147, 148 and 149 which in turn close switches 150, 151 and 152 when a photo-electric current is flowing. Due to this, the coils of relays 153, 154 and 155 are energized which open the already mentioned switches 125, 126 and 127 in conductor 35. Thus, a rolling pressure measured at pressure measuring device 25 could no longer have an effect on grid 129 of tube 130. For that reason, there are connected to grid 129 three adjustable grid resistors 43, 49 and 55 connected in parallel which are connected additionally in cascade arrangement to switches 155, 157 and 158 when photo-electric currents are flowing. The grid resistors 48, 49 and 541 obtain their voltage from a control resistor 159 in which a voltage is set which corresponds to the voltage increase from the pressure measuring device 26 determined at the indicator instrument 31 upon starting the operation. At the control resistor 159, the probable rolling pressure increase is thus preset upon initiating movement of a work piece in one path which influences the grid bias at grid 129 depending upon the number of resistors 48, :9 and 55 connected thereto.

In series with switches 15%, 151 and 152 are connected additional contacts 160, 161 and 162 which are opened by time-delay relays 163, 164 and 165. The time-delay relays are connected in parallel to the relays 153, 154 and 155 which actuate the switches 128 and 156, 126 and 157 and 127- and 158 simultaneously in pairs. As soon as the time lapse set on the time-delay relays has passed and the contacts 161), 161 and 152 are opened, relays 153, 154 and 155 also open, although photo-electric currents are still flowing and the switches 15%, 151 and 152 are closed.

Assuming that the rolling speed is twenty meters per second and the response time of the control arrangement upon initiating movement of a work piece is fifty milliseconds, the preset, hydraulic stretching force of the stay bolts must temporarily be decreased by the entire rolling pressure. With the assumed response time, the control command would thus be executed only after a work piece had covered a distance of one meter if the arrangement with the photo-cells 45 through 47' were not present. When the photo-cells for sensing the movement of the work piece arranged a distance of one meter from the roll frame, the execution of the control command for equalizing the rolling pressure increase coincides with the engagement of the work piece with the rolls which may be seen from the following description.

If no work pieces move through the roll frame, all the switches have the switching position illustrated in FIG. 5. The currents flowing through tubes 131) and 131 are in equilibrium or zero. When a leading or only work piece passes, for example, photo-cell 45, the switches 15th and 156 are closed and switch 125 is opened. Simultaneously the coil of the time-delay relay 163 is energized. In view of the fact that in disconnecting grid resistor 128 there remains connected in conductor 313 the grid resistor 48 with a voltage on grid 129 of tube 139 determined by the setting of resistor 159 which corresponds to a voltage which is increased correspondingly to the expected rolling pressure increase, the equilibrium of the currents flowing through tubes 130 and 131 is disturbed and the control starts to operate. The hydraulic stretching force has dropped off at the moment of initiating movement of a work piece an amount corresponding to the value predetermined by the setting of resistor 159. With the rolling pressure which now rises possible differences are regulated relative to the preset rolling pressure, in that with the initiation of movement of a work piece or shortly thereafter, the time-delay relay 163 opens the contact 160 so that relay 153 opens and the switches 125 and 155 again assume the positions illustrated in FIG. 5. Thus, grid 129 is again connected to conductor 30 and pressure measuring device 26, although the work piece path is still closed in the area of the photo-cell 45 and switch 150 and relay 153 remain disconnected until a new work piece is introduced into the same path of the roll frame. If previously the end of the work piece had passed photo-cell 45 and the photoelectric current ceases, switch opens, the time delay relay 163 is de-energized and the contact 165 closes.

The other photo-cells 46 and 47 operate in the same manner and under certain circumstances all three may operate simultaneously.

Beforeoperation of the mill, hydraulic pressure is supplied iro-m the source 39 through the fluid pressure adjusting means 38 and the conduit 34' to the cylinder 12a of the stay bolt 3a to elongate such stay bolt within the elastic limit at a pre-determined stressing value and the adjusting device 43 is utilized to feed into the computer 32 such pre-determined stressing value which represents the maximum value for the force required to elongate the stay bolts within the elastic limit of the material. After the rolls 25a and 21a have been set by means of the mechanical adjusting devices 24, 22, 23, the rolling mill' is now operated to roll a work piece and the actual roll pressure obtaining during the rolling operation is measured by the measuring device 26 and such roll pressure is indicated on the visual indicator 31 and is also fed into the computer 32. The fluid pressure obtaining in the conduit 34 converted into the effective elongating force applied to the stay bolt 3a is indicated on the visual indicator 33 and is also fed intothe computer 32. T he computer 32' operates to compare the sum of the rolling pressure as supplied by the measuring device 26 and the elongating force supplied from the conduit 34 with the pre-determined stressing value supplied from the adjusting means 43 and the resulting output of the computer 32 as indicated on the visual indicator 41 determines any change necessary in the fluid pressure applied through the conduit 34 to the cylinder 12a to compensate for any changes of the sum of the rolling pressure and elongating force stressing value. Such variation, if any, is conducted through the conductor 35, switch 36, visual indicator 37 and switch 42 to the fluid pressure adjusting means 38' wherein suitable adjustment is made in the fluid pressure supplied to the cylinder 12a to compensate for such dilferences, which would result in undesirable changes of the stressing forces in the stay bolts during rolling. Constant stressing forces, however, result in providing a constant roll gap. Since the only movement necessary to compensate for such differences occurs in elongation or contraction of the stay bolt 3a, there are no frictional forces to overcome and the inertia forces are practically negligible. Further the measuring information for the control system is obtained in the area of the roll gap, namely, by a measuring device 26, and not at a distance from a thickness gauge device. Consequently, compensating adjustments may take place almost instantaneously resulting in immediate adjustment of the rolling pressure to maintain the accuracy of the rolled work pieces.

In order to provide rapid compensation for bending or distortion. of the rolls and associated equipment, any change in the roll gap resulting therefrom is detected by the pressure measuring device 26 and such changes are fed into the second computer 44 and the output of such computer which is connected to the adjusting device 43 operates to vary the adjusting device 43 as a function of the roll bending to feed a new or corrected lowest predetermined stressing value into the first computer 32. As a result, this information supplied to the computer 32 permits operation thereof in a manner to actuate the fluid pressure adjusting means 38 to adjust the fluid pressure supplied to the cylinder 12a thereby permitting elongation or contraction of the stay bolt 3a in a manner to substantially instantaneously compensate for changes in roll bending conditions and thereby maintain accuracy of the rolled work pieces.

In the rolling of single or continuous work pieces, the control operation, above described, is all that is necessary in order to satisfactorily maintain the accuracy of such rolled work pieces. However, where it is desired to feed work pieces successively to the rolls, one piece being fed while the previous piece is in the process of rolling and so on depending upon the number of work pieces simultanously rolled in succession, it is necessary to provide additional control means which may take the form of the photo-electric devices 45, 46 and 47 which serve to detect the approach of the leading end of the work piece about to enter between the rolls. The approach of such work piece or work pieces is signalled to the photo-electric control circuits 4:8, 439 and 50 which operate through the visual indicator 37 and switch 42 to supply such information of an approaching work piece to the fluid pressure adjusting means 38 which will operate to pre-adjust the fluid pressure supplied to the cylinder 12a to lower the value thereof and therefore, to contract the stay bolts in such a manner as to compensate for the shock occasioned by each work piece entering between the rolls. The distance between the photo-electric devices 45, 46 and 47 should be chosen in accordance with the rolling velocity so as to compensate for the delay in response of the control system. This control function is entirely automatic and in View of the nature of the controlling means, connected to the control system as described above, the compensating adjustments are substantially instantaneous and result in providing adequate control of the accuracy of the rolled work pieces.

If desired, either or both of the switches 36 and 42 may be opened to disconnect the controlling system from the fluid pressure adjusting means 38, in which case the control system, by means of the visual indicators connected therein, serves as an indicating system to provide visual information to an operator as to variations in the rolling pressure as compared to the predetermined stressing value and also to indicate the corrections necessary to compensate for such variations. It is, of course, understood that compensating adjustments may be manually made in accordance with the information visually supplied as indicated above, but in normal use, automatic operation of the fluid pressure adjusting means 38 to automatically compensate for variations is utilized and is desirable, since manual compensating adjustments cannot be made at a sufiiciently rapid rate to maintain work piece accuracy, but are normally utilized for initial adjustment of the apparatus.

It is, of course, understood that in setting up the apparatus tor use, that with the structure shown in FIG. 3, the initial spacing of the rolls is determined by adjustment of the inter-engaging threaded means 22 and 23, while in the structure shown in FIGS. 1 and 2, initial adjustment of the roll spacing is accomplished by utilizing the adjusting screws 8 and 8a. The application of the automatic control system described above to the structures shown in FIGS. 1 and 2 is the same as described in connection with FIG. 3, the only difference being in the particular manner in which the initial adjustment of the roll is accomplished and in the particular structure of the means for elongating or contracting the stay bolt as shown in FIG. 2.

I With reference to FIG. 4, there is shown a frameless mill stand according to FIG. 3 for rolling from the right to the left, and a hydraulically operated control system. In so far as details of the control system according to FIG. 3 agree with the diagrammatic representation the same numeral-s have been utilised :for equivalent means.

The most vital part of the hydraulically operated control system is a hydraulic relay 641i similar to the relay disclosed in German Patent No. 634,325 and this relay performs the functions of the electronic computer 32 and the fluid pressure adjusting means 38 according to FIG. 3. Thus the relay 60 is a combined controlling and fluid pressure adjusting member. In detail the relay 6% is designed as follows: in a tube-shaped housing there are three outer ring channels '611, 62 and 63 and three inner ring channels 54, 65 and 66, located in pairs one opposite the other and connected by bores. Through an outer connection *67 with radial bores 68, 69 and 70 the outer ring channels are connected with conduits i.e. the pressure conduit 39 coming from a hydraulic accumulator 39a is connected with the ring channels 63 and '66 by bore 70, whereas from bore 68 with ring channels 61 and 64 an outlet 71 leads to a container 72. From container 72, the fluid is forced into accumulator 39a by means of a continuously operating pump 73 whose constant pressure is maintained in known manner in that pump 73 discharges into outlet 71 through a spring-loaded =valve '74, if the fluid in accumulator 39a has the predetermined pressure. Moreover, a conduit 40 connected to bore 69 and ring channels 62. and 55 leads to a multiplier 75 in which the fluid pressure supplied by accumulator 39a is increased. A high-pressure chamber 76 of the multiplier 75 is connected to a conduit 34 which leads to the enlarged portions 317 of the stay bolts 3a, i.e. into their cylinders 12a (see FIG. 3).

Preferably each pair of stay bolts of each side of the roll stand is connected by itself through conduits 34 to one control system.

In the fluid pressure adjusting means of relay 66 there is provided a two-part tube-shaped piston 38a and 3%, which is made in two parts only tor reasons of manufacture, and a smaller adjusting piston 38c, disposed in a chamber 77. Chamber 77 is connected to high-pressure conduit 34- through conduit 7 8 which virtually represents the return of the control circuit.

The controlling effect proper is obtained by a control piston 32b sliding in the pressure adjusting means 38b, 38a. A piston 32a acts upon the one tront surface of center piston 32b and is affected by the changing pressure in a chamber 7 9. Chamber 79 is connected through a conduit 3% to a pair of hydraulically operated pickups 26 of one stand side. Any change of rolling pressure recorded by the pickups results in a changing pressure in pressure chamber 79, whereby control pistons 32a, 32b are adjusted relatively to pressure adjusting means 38a, 38b under the action of a restoring spring 80.

Control piston 38b acts together with the inner ring channels and bores of pressure adjusting piston 38a, 35b by means of leading edges in such a manner that in case of a relative movement of control piston 32b to the left, pressure conduit 39 is switched to conduit '40 and multiplier 75 in order to increase the pressure in conduit 34 and to elongate the stay bolts 3a. A movement of control piston 32b to the left however opens conduit 40 to outlet 71 to contract the stay bolts.

To adjust the predetermined stressing value, an adjusting means 43 with a compression spring 81 which should be as inertialess as possible and which is lgripped between two caps 82 and '83 is used. Cap -33 is adjustable through a spindle 8'4- and a worm gear 85, 86 by a motor 87. The other cap 82 acts through a piston '88 upon a roller '89 of a two-armed lever which pivots about the axle 91, embraces with its forked arm 96a the housing of relay 60' and which acts by means of two semi-annular parts 92 and 93 upon one front surface of pressure-adjusting pistons 38a, 38b and thus also upon piston 38c.

The pivoting axle 91 is located in a slide 94 which is guided in a slot 95 of lever '99. In a simple model of the control system described, the position of slide 95 with pivoting axle 9.1 can be fixed at difierent leverages of the two-armed lever 90, in order to change the effective elastic force of spring 81. However, the representation shows an automatic changing of this leverage depending on the rolling pressure so as to compensate for roll bending. For this purpose a further two-armed lever 96 is provided which pivots about an axle 97 and embraces with its forked end the pivoting axle 91 of lever '90. The other free end of lever 96 is connected to a piston '99 of an adjusting member 44- through a link 98. The pressure chamber of the adjusting member 44 is in connection with conduit 30 coming from the pickups 26. If the rolling pressure is diminished, piston 99 is retracted by a spring 10 1.

In starting the operation of the control system and the roliing mill, hydraulic pressure is supplied from accumulator 359a and conduit 39 to the hydraulic relay 6t) and, as no rolling pressure, nor hydraulic pressure exists in the chambers 79 and 77, respectively, control piston 32!) is in its most left position relative to piston 38a and 3811, under the elastic forces of the springs 80 and 81, respectively. Thereby conduit 39 is open to conduit 40 so that high fluid pressure is built up in conduit 34 so as to elongate the stay bolts 3a to a predetermined value stressing.

The initial elongation of the stay bolts 3a is predetermined by the elastic force of spring 31, acting upon pressure-adjusting pistons 33a, 38b and piston 380 in direction of chamber 77 in that a balanced condition will be attained between the increasing load acting upon the piston 380 from the right in accordance with the increasing fluid pressure in conduit 34 and 78 and in chamber '77, and the predetermined elastic force from the spring 81, acting upon the adjusting pistons 38a, 33b and piston 380 from the left. In the balanced condition the pressure-adjusting pistons have been moved'to the left so that ring channels 62 and d5 and bore 69, corresponding to conduit iii, are closed by the leading edges of control piston 32!).

After the roll gap has been adjusted by the device 22, 23, 24, 25, and 9!; (see FIG. 3) so that the distance X is fixed, rolling can start. In the drawing, the control and adjusting members of relay tl are shown in a state of balance, i.e. in the state where all conduits 39, 71 and- A are closed. It must be mentioned that here is a second balanced condition between the spring 3t) acting upon the control piston 32]) and a certain initial fluid pressure in the chamber 79 and the hydraulic pickups 26, even if a rolling-pressure is not as yet present.

When starting rolling operations, the rolling-pressure is suddenly increased which would result in an undesirable increase of elongation of the stay bolts and the distance X, and thus of the roll gap. At the very moment when an actual rolling pressure occurs during the rolling operation, pistons 32a and 32]) move to the right in accorclance with increasing pressure in conduit 30 and chamber 79, thus opening conduit 4% to outlet 71. Now the high fluid pressure in conduits 34 and 78 decreases, the stay bolts 3a being contracted to the initial length, so as to maintain the initial distance X, i.e. to keep the roll gap constant. The movement of the control piston 32b to the right, ends it the pressure adjusting pistons 38a, 38b follow in the same direction, because the fluid pressure in chamber '77 is decreased, till in a new state of balance both the piston 32b and the piston 38a occupy the initial position with respect to each other, in which conduits 4t) and 71 are closed.

f in the process of rolling the actual rolling-pressure is diminished, control piston 32b-moves to the left, opening conduit 39 against conduit 44? increasing the high fluid pressure in conduit 34' and thus elongating or stretching the stay bolts 3a, till pressure-adjusting piston 38a follows to the left, attaining the initial position relative to the control piston unit in which conduits 3% and 44 are closed.

As described above it was presumed that any constant value of the distance X is equivalent to the corresponding constant value of the roll gap. This is the case provided the rolls are sufliciently deflection-resistant. Then the pivoting axle 91 of the two-armed lever 9@ can be fixed. But, if roll-deflection shall be eliminated, the distance X must be reduced as the actual rolling-pressure increases, and must be increased as the actual rolling-pressure decreases. This will be effected by correcting the eflective elastic force of the spring 81 by changing the leverage of the two arms of lever 99 as already described. In more detail: a greater actual rolling-pressure results in a lifting of piston 99 in adjusting member 44 and a lowering of effective pivoting axle 91 in order to diminish the elastic force of spring 81 acting upon adjusting piston 38a. If, under the simultaneous control effect already described, control piston 32b moves to the right, a greater decrease of the high fluid pressure in conduits 34 and 78 results, till a new state of balance between the elastic force of the spring 81 and the load upon the piston She has been attained. This means that the stay bolts 3a contract more than necessary in view of constancy of the distance X. The dimensions are such that according to the roll spring constant, the elastic force of the spring 31 is automatically corrected by adjusting member 44 depending on the varying deflection of the rolls.

There are other influences in rolling conditions which should be eliminated,.especially in hot and cold rolling of strips, e.g. variations of roll diameter by abrasion or heating. Since these variations occur slowly, it is not practical to eliminate them by the described control, since a lowering rolling pressure by reason of abrasion of roll diameters would result in an undesirable elongation of the stay bolts instead of a contraction. Rather, a thickness gauge is provided behind the stand for this purpose, the output of which causes impulses in an electric relay 1% to control motor 87. The thickness gauge measuring device may be of the kind described in German patent No. 807,853. As already described, motor 87 drives through the worm gear 85, 86 to adjust cap 83 and to correct the initial elastic force of spring 81. If the roll diameter is reduced by abrasion work piece thickness increases. The rolls must be screwed down by contraction of the stay bolts, that means by diminishing the high fluid pressure in conduit 34. This is accomplished by starting the motor 87 in such a sense of rotation that cap 83 is shifted from cap 82 whereby equivalent to the lowering of pivoting axle 91, a balance in the control system is adjusted under a diminished high fluid pressure in the conduits 34 and 78. Motor 87 is stopped if the thickness gauge 105 does not detect any variation from the required thickness. of the strip.

The highest demands are made on the control system when a work piece enters the roll gap. The occurring shocks may entail delay in the course of control operation. It more than one Worked piece is rolled simultaneously, each initial pass of the work piece heads would result in a temporary thickening not onlyof the work piece head itself, but also of a-work piece already in the roll gap, viz. by reason of response time of the control system. To. eliminate the response time delay to the greatest extent possible, photo electric devices 45, 46 and 47 are located for each of, say, three lines of work pieces, in front of the stand. The photo-electric devices serve to detect the approach, of the leading end of the work pieces about to enter between the rolls. Electric control circuits 48, 49 and 50 as described above are provided which control magnetic valves 110, 1111 and 112 by electric lines 113, 114 and 115. The magnetic valves are located on a hydraulic adjusting multiplier 116 with a three-step low-pressure piston 117, whose high pressure piston 118 protrudes into an enclosed pressure chamber 119. From chamber 11%, a. conduit 120. leads to a cylinder 121 of a further adjusting member, containing a piston 122 which is connected with the piston $9 of the already described adjusting member 44. It is intended by this assembly, to initiate the adjustments of piston 9? before the suddenly increased rolling pressure is signalled in order to contract the stay bolts for a predetermined size. This is accomplished in that the magnetic valves Ht), 111 and 112 open several conduits in multiplier 116, when the photo-electric devices 45, 45 or 47 detect the approach of a leading end. Then, throughconduit 123, the accumulator fluid pressure is admitted to the varying low-pressure surfaces of piston 11-7. Not shown are means of pressure relief for the three pressure chambers of piston 1-127, when the final signal arrives at adjusting member 44 through conduit 3%.

It will be seen that by the above described invention there has been provided a highly effective control system for accurately and rapidly controlling the operation of a rolling mill in accordance with changes in roll pressure to automatically and substantially instantaneously compensate for changing conditions which might adversely affect the accuracy of the rolled pieces, the control system operating to automatically compensate for such changes or variations in a manner resulting in maintaining a high degree of accuracy in the rolled work pieces.

While the control system has been described as opering in response to changes in roll pressure, it is also entirely possible to operate the system in response to variations in dimensions of the rolled work pieces and for this purpose, a work piece measuring device may be provided following the roll to measure the rolled work pieces and the measuring device is connected to the second computer 44 in such a manner that variations in dimensions of the rolled work pieces will, through the computer 44, operate to actuate the adjusting device 43 to supply a corrected roll pressure value to the first computer 32 resulting in compensating for such dimensional variation.

It will be obvious to those skilled in the art that various changes may be made in the invention without departing from the spirit and scope thereof and therefore the invention is not limited by that shown in the drawing and described in the specification, but only as indicated in the appended claims.

What is claimed is:

1. A rolling mill including an upper roll and a lower roll, an upper frame member, said upper roll being rotatably carried by said frame member, a lower frame member, said lower roll being rotatably carried by said lower frame member, yieldable means for maintaining said upper and lower frame members in spaced relationship, stay bolts connecting said upper and lower frame members and being dimensioned to absorb the rolling pressure transmitted from said rolls, fluid pressure operated means operatively associated with said stay bolts to elongate the same before operation of the rolls within the elastic limit of the material of said stay bolts at a predetermined stressing value higher than the maximum rolling pressure, so that in operation the actual roll pressure causes additional elongation of of said stay bolts, and a fluid pressure adjusting means connected between a source of fluid pressure and said fluid pressure operated means, whereby the elongation of said stay bolts will be varied in accordance with variations in roll pressure to maintain the accuracy of work pieces being rolled, in that the sum of the actual roll pressure and the actual elongating force and thus the stress within said stay bolts is maintained substantially constant.

2. A rolling mill including an upper roll and a lower roll, an upper frame member, said upper roll being rotatably carried by said upper frame member, a lower frame member, said lower roll being rotatably carried by said lower frame member, yieldable means for maintaining said lower frame members in spaced relationship, stay bolts connecting said upper and lower frame members in a manner to absorb the rolling pressure transmitted from said rolls, fluid pressure operated means operatively associated with at least certain of said stay bolts to elongate the same within the elastic limit of the material of said stay bolts and a control system for controlling the pressure of fluid supplied to said fluid pressure operated means, said control system including a main control device, means for measuring the rolling pressure exerted on said stay bolts and for feeding such measurement to such control device, means for feeding the effective value of fluid pressure supplied to said fluid pressure operated means to said control device, and adjusting device for adjusting said control device in accordance with a predetermined stay bolt stressing value and a variable fluid pressure supply means for said fluid pressure operated means, said control device operating to control the operation of said fluid pressure supply means, whereby the elongation of said stay bolts will be varied in accordance with variations in roll pressure to maintain the accuracy of work pieces being 1d rolled, said control device serving to compare the sum of the actual roll pressure and actual stay bolt elongating force with the predetermined stressing value and by controlling the operation of said supply means to maintain said sum substantially constant.

3. A rolling mill including an upper roll and a lower roll, an upper frame member, said upper roll being rotatably carried by said upper frame member, a lower frame member, said lower roll being rotatably carried by said lower frame member, yieldable means for maintaining said upper and lower frame members in spaced relationship, stay bolts connecting said upper and lower frame members in a manner to absorb the rolling pressure transmitted from said rolls, fluid pressure operated means operati-vely associated with at least certain of said stay bolts to elongate the same within the elastic limit of the material of said stay bolts and a control system for controlling the pressure of fluid supplied to said fluid pressure operated means, said control system including a hydraulic relay, a substantially constant fluid pressure source connected to said relay, a fluid pressure multiplying means connected to said relay and to said fluid pressure operated means, means for feeding the effective value of fluid pressure supplied to said fluid pressure operated means to said relay, means for measuring the rolling pressure exerted on said stay bolts and for feeding such measurement to such relay, an adjusting device for feeding a predetermined stressing value to said relay and means connecting said measuring means to said adjusting device, whereby said relay will operate to compare the sum of the actual roll pressure and actual stay bolt elongating force with the predetermined stressing value and by controlling the operation of said fluid pressure multiplying means maintain said sum substantially constant, thereby varying the elongation of said stay bolts in accordance with variations in roll pressure to maintain the accuracy of work pieces being rolled.

4. A rolling mill as defined in claim 3, in which a plurality of photo-electric devices are disposed in advance of said rolls in the path of movement of work pieces entering said rolls, photo-electric control circuits connected to said photo-electric devices, a second fluid pressure multiplying means connected to said fluid pressure source and to said adjusting device and valves actuated by said control circuits for controlling the operation of said second fluid pressure multiplying means, whereby the leading ends of work pieces entering said rolls will actuate said photoelectric devices to vary the elongation of said stay bolts in the moment of work pieces entering said rolls.

5. A rolling mill as defined in claim 3, in which a work piece thickness gauge is disposed following said rolls to measure a rolled work piece, said gauge being connected to said adjusting device to adjust said relay in accordance with the dimensions of a rolled work piece.

6. A rolling mill including an upper roll and a lower roll, an upper frame member, said upper roll being rotatably carried by said upper frame member, a lower frame member, said lower roll being rotatably carried by said lower frame member, yieldable means for maintaining said upper and lower frame members in spaced relationship, stay bolts connecting said upper and lower frame members and being dimensioned to absorb the rolling pressure transmitted from said rolls, fluid pressure operated means operatively associated with said stay bolts to elongate the same before operation of the rolls within the elastic limit of the material of said stay bolts at a predetermined stressing value higher than the maximum rolling pressure, and a control system for controlling the pressure of fluid supplied to said fluid pressure operated means, said control system including a computer, means for measuring the rolling pressure exerted on said stay bolts and for feeding such measurement to said computer, means for feeding the effective value of fluid pressure supplied to said fluid pressure operated means to said computer, an

adjusting device for feeding a pre-determined stay bolt stressing value to said computer, a fluid pressure adjusting means connected between a source of fluid pressure and said fluid pressure operated means, the output of said computer being connected to said fluid pressure adjusting means, whereby the elongation of said stay bolts will be varied in accordance with variations in roll pressure to maintain the accuracy of Work pieces being rolled, in that the sum of the actual roll pressure and the actual elongating force is compared with the predetermined stressing value and maintained constant.

7. A rolling mill as defined in claim 6, in which a second computer is provided, means connecting said second computer to said measuring means and to said adjusting device for feeding the predetermined stressing value to said first computer, said second computer serving to de termine a roll bending value based on the actual roll pressure in said mill and to actuate said adjusting device to feed a corrected predetermined stressing value to said first computer, whereby the elongation of said stay bolts will be varied in accordance with variations in roll pressure to maintain the accuracy of Work pieces being rolled, in that the sum of the actual roll pressure and the actual elongating force is compared with the corrected predetermined stressing value and maintained constant.

8. A rolling mil-l including an upper roll and a lower roll, an upper frame member, said upper roll being rotatably carried by said upper frame member, a lower frame member, said lower roll being rotatably carried by said lower frame member, yieldable means for maintaining said upper and lower frame members in spaced relationship, stay bolts connecting said upper and lower frame members in a manner to absorb the rolling pressure transmitted from said rolls, fluid pressure operated means operatively associated with at least certain of said stay bolts to elongate the same Within the elastic limit of the material of said stay bolts and a control system for controlling the pressure of fluid supplied to said fluid pressure operated means, said control system including an electronic or hydraulic computer, means for measuring the rolling pressure exerted on said stay bolts and for feeding such measurement to said computer, means for feeding the effective value of fluid pressure supplied to said fluid pressure operated means to said computer, an adjusting device for feeding a pre-determined stay bolt stressing value to said computer, a second electronic or hydraulic computer, means connecting said second computer to said is measuring means and to said adjusting device, said second computer serving to determine a roll bending value based on the actual roll pressure in said mill and to actuate said adjusting device to feed a corrected stressing value to said first computer and a fluid pressure adjusting means connected between a source of fluid pressure and said fluid pressure operated means, the output of said first computer being connected to said fluid pressure adjusting means.v

whereby the elongation of said: stay bolts will be varied in accordance with variations in roll pressure to maintain the accuracy of work pieces being rolled.

9. A rolling mill as defined in claim 8, in which a plurality of photo-electric devices are disposed in advance of said rolls in the path of movement of work pieces entering said rolls and photo-electric control circuits connected to said photo-electric devices and to said fluid pressure adjusting means, whereby the leading ends of work pieces entering said rolls will actuate said photoelectric devices to vary the elongation of said stay bolts at the moment at which work pieces enter said rolls.

10. A rolling mill as defined in claim 8, in which said fluid pressure operated means comprises a cylinder in one of said frame members surrounding a stay bolt, a tubular piston slidably received on said stay bolt and in said cylinder and means on said piston engaging the other of said frame members.

11. A rolling mill as defined in claim 8, in which a work piece measuring device is disposed following said rolls to measure the rolled work piece, such measurement being fed to said adjusting means to feed a corrected stay bolt stressing value to said first computer in accordance with changes of the dimensions of a rolled work piece.

References Cited in the file of this patent UNITED STATES PATENTS 1,935,091 Averson Nov. 14, 1933 2,523,553 Blain Sept. 26, 1950 2,680,978 Hessenberg et al June 15, 1954 2,934,968 Nuemann et al May 3, 1960 2,985,043 Roberts May 23, 1961 FOREIGN PATENTS 214,663 Australia June 15, 1956 571,793 Canada Mar. 3, 1959 1,045,266 France June 24, 1953 137,772 Sweden Oct. 28, 1952 

1. A ROLLING MILL INCLUDING AN UPPER ROLL AND A LOWER ROLL, AN UPPER FRAME MEMBER, SAID UPPER ROLL BEING ROTATABLY CARRIED BY SAID FRAME MEMBER, A LOWER FRAME MEMBER, SAID LOWER ROLL BEING ROTATABLY CARRIED BY SAID LOWER FRAME MEMBER, YIELDABLE MEANS FOR MAINTAINING SAID UPPER AND LOWER FRAME MEMBERS IN SPACED RELATIONSHIP, STAY BOLTS CONNECTING SAID UPPER AND LOWER FRAME MEMBERS AND BEING DIMENSIONED TO ABSORB THE ROLLING PRESSURE TRANSMITTED FROM SAID ROLLS, FLUID PRESSURE OPERATED MEANS OPERATIVELY ASSOCIATED WITH SAID STAY BOLTS TO ELONGATE THE SAME BEFORE OPERATION OF THE ROLLS WITHIN THE ELASTIC LIMIT OF THE MATERIAL OF SAID STAY BOLTS AT A PREDETERMINED STRESSING VALUE HIGHER THAN THE MAXIMUM ROLLING PRESSURE, SO THAT IN OPERATION THE ACTUAL ROLL PRESSURE CAUSES ADDITIONAL ELONGATION OF OF SAID STAY BOLTS, AND A FLUID PRESSURE ADJUSTING MEANS CONNECTED BETWEEN A SOURCE OF FLUID PRESSURE AND SAID FLUID PRESSURE OPERATED MEANS, WHEREBY THE ELONGATION OF SAID STAY BOLTS WILL BE VARIED IN ACCORDANCE WITH VARIATIONS IN ROLL PRESSURE TO MAINTAIN THE ACCURACY OF WORK PIECES BEING ROLLED, IN THAT THE SUM OF THE ACTUAL ROLL PRESSURE AND THE ACTUAL ELONGATING FORCE AND THUS THE STRESS WITHIN SAID STAY BOLTS IS MAINTAINED SUBSTANTIALLY CONSTANT. 